1. Field of the Invention
This invention relates generally to radar (“radio detection and ranging”), and more specifically relates to down-converting reflected radio-frequency energy to audio output. One application of the invention is, for example, using radar to assist the visually impaired.
A method of frequency down-converting a radio frequency (RF) carrier termed ultra-wide-band (‘UWB’) was invented by McEwan et al. at Lawrence Livermore National Laboratories (LLNL) years ago (See U.S. Pat. Nos. 5,345,471, 5,361,070, and 5,661,385). This technique, herein termed “integrated sampling,” is being widely licensed for commercial use of the technique for multiple radar-related purposes. The basis of this integrated sampling technique was originally developed earlier in conjunction with the development of sampling oscilloscope technology. In more modern terms, the technique can be viewed as a form of “digital downconversion” where subsampling is used to alias the original spectrum down to a much lower frequency spectrum. In the case of the LLNL technology, the technique retains much of the older sampling oscilloscope technology in that the sampler is purely analog in nature with no analog-to-digital conversion employed.
The LLNL integrated sampling technique differs from more conventional methods of radar signal processing in that the RF carrier is directly down-converted to an intermediate frequency (IF) without the conventional use of mixer or local oscillator components. As long as the radar echo is consistent during the integrated sampling time period (millions of pulses and samples per second), the time-domain nature of the original RF carrier is accurately duplicated, for example, at the IF frequency, typically selected to be in the audio frequency range. This means that the “nature” of the radar echo is preserved upon down-conversion. The complete amplitude, phase, and Doppler characteristics of the echoed waveform are preserved and time-scaled, including propagation delay. Thus, by this technique, the speed of light propagation of the RF carrier scales down to approximately speed-of-sound delays when down-converted via integrated sampling.
2. Related Art                U.S. Pat. No. 3,626,416 (Rabow): This patent mentions conversion of received RF signals in two receivers to audio for presentation to human ears.                    The patent mentions only heterodyning conversion;            The patent discloses only direction finding for an external radio frequency source, and does not mention radar echoes; and,            The patent does not mention using the device for visual assistance.            Relative to the present invention, disadvantages of this disclosed system are:                            Echo time differential to each receiver is at the speed of light, and this differential is maintained through-out the system; and,                No provision for self-generation of the RF signal, that is, radar echo, is made.                                                U.S. Pat. No. 3,940,769 (Sherman, et al.): This patent mentions conversion of an RF radar echo into binaural audio for the purposes of allowing an operator to listen to the radar echo.                    The patent discloses a very specific means of processing the two received audio signals in terms of “sum” and “difference” audio channels derived from left side and right side radar receivers. Both channels are presented to each ear with one ear receiving the “sum channel” plus the “difference channel” and the other ear receiving the “sum channel” plus a filtered and inverted “difference channel”. The purpose of the patent is to enhance the angular difference in terms of audio amplitude, and a long derivation of proper filtering parameters is made in it;            The patent does not mention using the technique for visual assistance; and,            In the patent no specific method of converting the RF signal to audio is made, using the patent phrase “well-known methods of detection, demodulation, or heterodyning.”            Relative to the present invention, disadvantages of this disclosed system are:                            Echo time differential to each receiver is at the speed of light, and this differential is maintained through-out the system; and,                The signal at each ear is a processed vectoral sum of two audio signals, with the original echo content being perturbed and blended.                                                U.S. Pat. No. 4,280,204 (Elchinger): This patent mentions use of an ultrasonic device mounted onto a conventional mobility cane for assistance of the blind. The disclosed ultrasonic range finder can be configured for variable range and direction pointing.                    The patent discloses the ultrasonic echo being measured for range, and a separately created tone is sent to the user to indicate range.            Relative to the present invention, disadvantages of this disclosed system are:                            The original ultrasonic echo is not sent to the user to allow echo content to be used as a part of the information signal;                The disclosed device is cumbersome in that the user must configure and reconfigure it to point in a desired direction and to set a desired range; and,                In general, ultrasonic echo location suffers from several drawbacks:                                    Limited useful range of less than about 20 feet;                    Poor performance in high wind, rain and snow;                    Subject to ultrasonic noise interference; and,                    Narrow coverage zone for any single transducer.                                                                                U.S. Pat. No. 4,761,770 (Kim, et al.): This patent mentions presenting a binaural echo signal to the user's ears using two ultrasonic receivers.                    The signal is “downconverted” from ultrasonic to sub-audio via “bucket-brigade” (BB) circuitry to time-stretch the signal. The original echo is sampled into the BB device at one clock-rate, and then extracted from the BB device using a much slower clock rate.                            The time-stretch is purposely configured to place the downconverted ultrasonic signal into the sub-audio range so that time delays in echoes are stretched to more discernable delays as a function of distance and human ability to discern delay; and,                The sub-audio signal is then modulated with a white-noise signal to create an audible sound for the user.                                    Relative to the present invention, disadvantages of this disclosed system are:                            The original ultrasonic echo is distorted by the extreme time-stretching to sub-audio and then modulated with white noise. Therefore, the original true echo content is lost; and,                The system is based upon ultrasonics and suffers from the general drawbacks of ultrasonics noted above regarding the Elchinger reference.                                                U.S. Pat. No. 5,107,467 (Jorgensen, et al.): This patent mentions presenting two receivers to provide an echo signal to the user's ears, based upon ultrasonics.                    The original echo signal is peak detected and sampled via analog-to-digital conversion. It is then digitally delayed and stretched in accordance with range via a non-linear stretching algorithm. A processed signal called an “echo profile” is recreated at audio frequency via digital-to-analog conversion using the variable clock in accordance with the variable delay and time-stretching algorithm.            Relative to the present invention, disadvantages of this disclosed system are:                            The original ultrasonic echo is distorted by peak detector, A/D conversion, and D/A conversion with non-linear time stretching. Therefore, much of original true echo content is lost; and,                The system is based upon ultrasonics and suffers from the general drawbacks of ultrasonics noted above regarding the Elchinger and Kim, et al. references.                                                U.S. Pat. No. 6,671,226 (Finkel, et al.): This patent mentions using multiple ultrasonic range finders positioned at various angles on the chest area of the user to try and cover a broad area.                    The signal sent to the user is manufactured based upon echo-range response in each ultrasonic transceiver. Each transceiver is sequentially activated, and if a target is sensed for that transceiver a tone unique to each transceiver is sent to the user.            Relative to the present invention, disadvantages of this disclosed system are:                            The original ultrasonic echo is not preserved;                The system is cumbersome due to the large number of transceivers pointing in different directions, with each transceiver having its own tonal output; and,                The system is based upon ultrasonics and suffers from the general drawbacks of ultrasonics noted above regarding the Elchinger, Kim, et al., and Jorgensen, et al. references.                                                